Source : Webster's Revised Unabridged Dictionary (1913)

Tremolo \Tre"mo*lo\, n. [It. Cf. {Tremulous}.] (Mus.)
   (a) The rapid reiteration of tones without any apparent
       cessation, so as to produce a tremulous effect.
   (b) A certain contrivance in an organ, which causes the notes
       to sound with rapid pulses or beats, producing a
       tremulous effect; -- called also {tremolant}, and
       {tremulant}.

Source : WordNet®

tremolo
     n 1: (music) a tremulous effect produced by rapid repetition of a
          single tone or rapid alternation of two tones
     2: vocal vibrato especially an excessive or poorly controlled
        one